1. Field of the Invention
The invention generally relates to a miniature electronic display. More specifically the invention relates to a miniature electronic display which provides a magnified and synthesized virtual image from a microdisplay using two stages of magnification optics and an intermediate image synthesizing optic.
2. Description of Related Art
A continuing objective in the field of electronics is the miniaturization of electronic devices. Most electronic devices include an electronic display. As a result, the miniaturization of electronic displays is critical to the production of a wide variety of miniaturized electronic devices.
The purpose of an electronic display is to provide the human eye with a visual image of certain information. This visual image may be provided as either a real image or a virtual image. A real image refers to an image which is observed directly by the unaided human eye. A photograph is an example of a real image. Electronic displays which provide a real image generally provide some form of display surface on which the real image is formed and viewed. Examples of electronic displays which provide real images include liquid crystal displays, CRT monitors, and projection screens.
A real image electronic display is shown in FIG. 1. In real image electronic displays, the image viewed by the user is an image created on or projected onto a display surface. As a result, the size of the display surface limits the size of the image that can be provided to the user. Miniaturized electronic devices, because of their compact size, have limited space for a display surface and therefore are only able to accommodate a relatively small display image.
A relatively small display image creates a series of problems for the person viewing the image. The human eye is only able to resolve a limited amount of detail in an image. Two objects can be resolved by the eye as separate objects only when they are separated by a certain minimum angle as measured from the eye. The unaided human eye can resolve objects with an angle of separation of approximately 1-2 arc minutes or more.
The human eye also has difficulty focusing on source objects at very short distances from the eye without causing eye strain. The near point for the unaided human eye is defined as the closest distance that the eye can focus on an object without causing eye strain. The near point for the unaided human eye is about 25 cm for an average middle aged adult. The near point is much longer for an older adult and can be as long as 100 cm.
The display surface of a real image electronic display is comprised of an array of small objects, called pixels, which form a real image on the display surface. For the reasons described above, the resolution of the unaided human eye and the near point of the eye determine the minimum pixel size that the unaided human eye can resolve. Thus, for an electronic display located at the near point for an average middle aged adult, the minimum separation between pixels that can be resolved by the unaided eye is about 75.times.10.sup.-4 cm, or 75 microns. In order to accommodate for variations in the human eye, the minimum separation between the pixels is preferably larger than the minimum resolvable pixel size.
For small real image electronic displays, if useful amounts of information are assembled from pixels of this size, the display must be viewed at a distance close to the near point, and, as a result, the user must focus his or her eyes on the small display. The need to focus on a small electronic display each time the electronic display is used creates discomfort and eventually, unwanted eye strain on the user. The problem of eye strain becomes particularly acute when the user is only intermittently focusing on the electronic display. It therefore is desirable to employ an electronic display which minimizes the user's need to refocus his or her eyes in order to view the electronic display.
Another important property of the human eye which determines the utility of an electronic display is the eye's angular field of view of an image. The eye can see over a field of view of up to 100 degrees. However, beyond 10-15 degrees from the center of the field, the resolution degrades significantly. A comfortable field of view for normal electronic display surfaces is typically in the range of 20-40 degrees. For real image displays, the field of view is defined as the ratio between the largest dimension of the display surface and the distance from the eye to the display. An example of a display surface with such a field of view would be a TV screen with a 100 cm diagonal viewed at 150 cm. The human eye compensates for the lower resolution at the edges of the display surface by scanning the eye across the display. The scanning of the eye is called eye roll. The eye roll moves the pupil of the eye. The typical distance for the motion of the pupil of an adult is about 1 cm.
An optical system can produce both real and virtual images. Several examples of electronic displays providing real images were discussed above. A real image exists at a given location when, if a viewing surface is positioned at this location, a real image can be observed by the unaided eye. Conversely, a virtual image is an image which, if a viewing surface were positioned at the location of the virtual image, no image would be observed by the eye. An example of a virtual image is the image of fine print viewed through a magnifying glass. The print not only appears larger, it also appears to be located substantially behind the surface where the print actually exists. By definition, a virtual image can exist at a location where no display surface exists. The size of the virtual image therefore is not limited by the size of a display surface. Virtual image electronic displays thus have the advantage of eliminating the need for a large display surface in order to produce a large electronic image.
A virtual image electronic display must initially form a source object which is then imaged by an optical system to create the virtual image. A substantial advantage of a virtual image electronic display is that the source object initially created may be as small as can be usefully reimaged by the optical system. As a result, virtual image electronic displays may effectively utilize very small microdisplays to form the source object. Pixel sizes may be as small as a few microns in diameter, a size which the unaided eye cannot resolve. Rather, in order to view the source object formed by the microdisplay, substantial magnification of the optical system is required.
A virtual image must be created by an optical system of some kind. In a real image electronic display, it is the eye and the viewing surface properties which determine the viewing parameters. By contrast, in a virtual image display, the optical system determines most of the viewing parameters. When viewing a virtual image through an optical system, there are three important parameters relating to the ease of viewing the image. The first parameter is the range of distances from the eye which the optical system can be held and have the eye still see the entire virtual image. The second parameter is the apparent angular width of the virtual image which is commonly referred to as the field of view of the virtual image. The field of view is defined as the ratio of the apparent width of the virtual image to the apparent distance to the virtual image and is equivalent to the field of view for a real image display surface. The third parameter is the transverse distance that the eye may move with respect to the optical system and still have the eye see the entire virtual image through the optical system.
A simple magnifying lens is shown in FIG. 2. The function of a magnifying lens is to provide an image of a nearby object that is larger than the image seen by the unaided eye. The object is placed a distance from then simple lens that is less than the focal length of the lens. The eye observes a virtual image through the magnifying lens which is larger than the object itself. A simple magnifying lens can magnify a real display surface to produce a virtual image that is significantly larger that the real display. In addition, if the object is placed at the focal point for the magnifying lens, the apparent location of the image is very far away. As a result, the eye is able to view the virtual image in a very relaxed state, thereby minimizing the creation of eye strain on the user.
The far point of a lens refers to the longest distance that the eye can be held from a magnifying lens and still see the entire virtual image. As illustrated in FIG. 3, the far point is related to the field of view of the virtual image, the eye roll, and the diameter of the optic. The far point increases as the diameter of the optic increases. Optical devices which provide a far point which is a short distance from the optic are undesirable due to the inconvenience and discomfort associated with placing the eye in close proximity with the optic. It is therefore preferred that an optic provide a long far point in order to enable the magnified image to be viewed through the optic at a comfortable and convenient range of distances from the optic.
For a simple magnifying lens, the maximum diameter that the optic can have is related to the magnification of the optic. In order to increase the magnification of an optic, the focal length of the optic must be decreased. According to simple lens physics, the maximum diameter that a simple optic can have is approximately equal to the focal length of the optic. Thus, in order to increase the magnification of an optic, the focal length of the optic must be reduced. This reduces the maximum diameter that the optic can have which, in turn, reduces the maximum eye relief provided by the optic. In addition, as the magnification of a simple lens is increased and the diameter of the optic is reduced, the amount of translational movement that the eye can have relative to the magnifying lens while still being able to see the image is reduced. Combined, these factors serve to limit the degree of magnification that a simple magnifying lens can provide.
For example, users with eye glasses generally require an eye relief of at least 25 mm. It is thus preferred that an electronic display provide an eye relief of at least about 25 mm. A simple 10.times. magnifying lens generally has a focal length of about 25 mm. The eye relief provided by a 10.times. magnifier viewing a virtual image with 40 degree field of view is about 25 mm. Since eye relief decreases as the magnification of an optic increases, magnifying lenses that provide greater than 10.times. magnification are not useful for providing eye relief of at least about 25 mm.
A compound microscope is depicted in FIG. 4. The simplest compound microscope is an optical system with two magnifying optics. The lens closest to the eye is referred to as the eyepiece. The lens closest to the source object is called the objective. The objective forms a real inverted, and usually magnified image of the object. This real image resides in space on the focal plane of the eyepiece. The eyepiece magnifies this real image even further. Compound microscopes have the advantage of providing a higher magnification of nearby objects than can be achieved using a simple magnifying lens.
Compound microscope optical systems have several disadvantages that arise from the path of the light rays through the optical system. The exit pupil of the compound microscope is defined as the transverse distance across the eyepiece where the entire image of the source object is still visible. When the pupil of the eye is outside the exit pupil, light rays from some part of the object are blocked by the optical system and the virtual image is vignetted. The light rays from the edge of the object must also intersect the pupil of the eye. If the eye is too close to the eyepiece, the edge of the object no longer appears illuminated since the light rays from the edge of the object are blocked by the optical system and the virtual image is vignetted. If the eye is too far away from the eye piece, again the light rays from the edge of the object are blocked and virtual image is vignetted. The point where the entire virtual image is visible in the compound microscope is called the eyepoint. The distance from the eyepoint to the eyepiece is commonly referred to as the eye relief and is equivalent to the far point of a simple magnifying lens. The volume of space around the eyepoint where the image is still visible is restricted by the optical system, and as a result, the functional volume of space within which the user's eye can be placed is greatly limited in a compound microscope. Given the limited functional volume provided by a compound magnification system, it is generally necessary to move the compound microscope to compensate for the user's eye movements. This greatly limits the functional utility of the compound microscope in viewing systems for electronic displays.
In view of the deficiencies associated with electronic displays using simple and compound magnification systems, a need currently exists for an inexpensive, miniature virtual image electronic display that is positionable within a small volume, that provides the user with a large field of view, a virtual image with a significant degree of eye relief and a large translational distance.